Tuning algorithm for multi-tap signal cancellation circuit

ABSTRACT

A self-interference signal cancellation circuit includes a transmitter for transmitting a transmit signal, a plurality of signal paths, a controller, and a receiver for receiving a signal. Each signal path includes a delay element and a variable attenuator having attenuation levels set by the controller. A combiner generates an output signal by combining outputs of the signal paths. The circuit computes a matrix based on first and second output signals associated with first and second attenuation levels. The controller concurrently varies the attenuation level of each signal path so that a product of the matrix and the attenuation levels of the signal paths is substantially equal to the received signal. The circuit may iteratively compute the matrix using different transmit signal frequencies or with an FFT. The controller iteratively varies the attenuation level of the attenuators until a sum of the product and the received signal satisfies a predefined condition.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 §U.S.C. 119(e) of U.S.Provisional Patent Application No. 61/754,447, filed Jan. 18, 2013,entitled “Tuning Algorithm for Multi-Tap Signal Cancellation Circuit”,the content of which is incorporated herein by reference in itsentirety.

The present application is related to application Ser. No. 14/106,664,filed Dec. 13, 2013, entitled “Feed Forward Signal Cancellation”, thecontent of which is incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to wireless communication, and moreparticularly to a full duplex wireless communication system.

BACKGROUND OF THE INVENTION

A wireless system often operates in a half-duplex mode to eithertransmit or receive data at any given time. A device operating in afull-duplex mode may simultaneously transmit and receive data. However,the simultaneous transmission and reception of data are carried out overdifferent frequencies. For example, a full-duplex cell phone uses afirst frequency for transmission and a second frequency for reception.As is well known, using the same frequency for simultaneous transmissionand reception in a conventional wireless system results in significantamount of self-interference at the receiver, thereby, rendering thesystem ineffective in receiving the desired signal.

BRIEF SUMMARY OF THE INVENTION

A circuit, in accordance with one embodiment of the present invention,includes a transmitter for transmitting a transmit signal at a selectedfrequency, a plurality of signal paths, a controller unit, and areceiver for receiving a signal. Each signal path includes a delayelement and a variable attenuator having attenuation levels that can beset by the controller unit. A combiner generates an output signal bycombining outputs of all the signal paths. The circuit computes a matrixbased on first and second output signals associated with first andsecond attenuation levels. The controller circuit concurrently variesthe attenuation level of each signal path so that a product of thematrix and the attenuation levels of the signal paths is substantiallyequal to the received signal.

In one embodiment, the circuit further includes, at least one antennafor receiving a signal. In one embodiment, each of the signal paths isadapted to receive a sample of a transmit signal and generate a delayedand weighted sample of the transmit signal. In one embodiment, thecircuit further includes a plurality of passive couplers adapted tochange a phase of one or more delayed and weighted signal paths inrelation to the transmit signal, and a coupler adapted to subtract theresult of the product from the receive signal. The circuit caniteratively compute the matrix using different transmit signalfrequencies or by taking a frequency domain representation of theresponse to a wideband transmit signal, such as taking the fast Fouriertransform.

In one embodiment, the circuit computes the matrix by applying awideband transmit signal to the signal paths and computing the Fouriertransform of the transmit signal and receive signal. The matrix may be acomplex M×N matrix, where M is the number of frequency bins in theFourier transform and N is the number of signal paths. The frequencydomain representation of the received signal may be a complex M×1 columnvector, and the attenuation level of each signal path is an element of areal N×1 column vector. In one embodiment, the product of the matrix andthe attenuation levels of the signal paths is a complex M×1 columnvector that is substantially equal to the received signal.

In one embodiment, the circuit further includes a circulator having afirst port coupled to an antenna, a second port coupled to a transmitline of the circuit, and a third port coupled to a receive line of thecircuit. In one embodiment, the second port of the circulator isdisconnected from the transmit line while the attenuation levels areset, output signals are obtained for the computation of the matrix. Inone embodiment, the third port of the circulator is connected to thereceive line and all attenuators are set to the maximum attenuationlevel while the signal is received by the receiver.

In one embodiment, the controller unit iteratively varies theattenuation level of each variable attenuator until a sum of the productand the received signal satisfies a predefined condition.

Embodiments of the present invention also provide a method forperforming a self-interference signal cancellation in a full-duplexwireless communication system with a cancellation circuit. The methodincludes supplying a sample of a transmit signal at a first frequency bya transmitter to the cancellation circuit. The cancellation circuitincludes a plurality of parallel signal paths, each of the signal pathsincludes a delay element and a variable attenuator. The method furtherincludes setting a first attenuation level to one or more of thevariable attenuators, obtaining a first output signal of thecancellation circuit, setting a second attenuation level to the one ormore of the variable attenuators, and obtaining a second output signalof the cancellation circuit. The method also includes generating amatrix based on the first and second output signals, receiving a signalat a receiver, and varying an attenuation level of each of the variableattenuators so that a product of the matrix and the attenuation levelsis substantially equal to the received signal.

The method, in accordance with one embodiment of the present invention,further includes, supplying the sample of the transmit signal at asecond frequency by the transmitter to the cancellation circuit, settingthe first attenuation level to the one or more of the variableattenuators, obtaining a third output signal of the cancellationcircuit, setting the second attenuation level to the one or more of thevariable attenuators, obtaining a fourth output signal of thecancellation circuit, and expanding the matrix based on the third andfourth output signals. The method can iteratively repeat the above stepswith a third, a fourth frequencies to expand the matrix.

In one embodiment, the matrix can be expanded to an M×N matrix, where Mis the number of transmit signal frequencies and N is the number ofsignal paths of the cancellation circuit.

In one embodiment, the first attenuation level is the maximumattenuation level and the second attenuation level is the minimumattenuation level. In one embodiment, the first and second attenuationlevels are applied to the variable attenuators of all of the signalpaths.

In one embodiment, the method further includes iteratively varying theattenuation level of each variable attenuator until a sum of the productand the received signal satisfied a predefined condition.

The following description, together with the accompanying drawings, willprovide a better understanding of the nature and advantages of theclaimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a full-duplex wirelesscommunication system, in accordance with one embodiment of the presentinvention.

FIG. 2 is a simplified block diagram of a full-duplex wirelesscommunication system, in accordance with one embodiment of the presentinvention.

FIG. 3 is a simplified block diagram of a full-duplex wirelesscommunication system, in accordance with one embodiment of the presentinvention.

FIG. 4 is a graph showing an exemplary response plot associated withdifferent frequencies and attenuation levels of the self-interferencecancellation circuit of FIG. 3, in accordance with one embodiment of thepresent invention.

FIG. 5 is a flowchart of a method for cancelling or reducing aself-interference signal, in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described more fullyhereinafter with reference to the accompanying drawings. This inventionmay, however, be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein. Like numbersrefer to like elements throughout.

It is understood that the term “vector” is a synonym for signal. Theterms “complex vector” and “real vector” are synonyms for complex signaland real signal, respectively. A vector can be an analog signalincluding an analog component I(f), Q(f), or a digital signal includinga digital component I(n), Q(n). A complex vector include acomplex-valued signal I+jQ. A column vector is an m×1 matrix, i.e., amatrix consisting of a single column of m elements. A m×n matrix is amatrix consisting of m rows and n columns.

FIG. 1 is a simplified block diagram of a full-duplex wirelesscommunication device 100, in accordance with one embodiment of thepresent invention. Wireless communication device 100, which may be acellular phone, a base station, an access point or the like, isconfigured to transmit data/signals via transmit antenna A11 and receivedata/signals via a receive antenna A12. Wireless communication device(herein alternatively referred to as device) 100 is also shown, asincluding, a transmit front-end 201, a signal splitter 210, a receivefront end 301, a signal combiner 310, and a self-interferencecancellation circuit 390. Device 100 may be compatible and operate inconformity with one or more communication standards such as WiFi™,Bluetooth®, GSM EDGE Radio Access Network (“GERAN”), UniversalTerrestrial Radio Access Network (“UTRAN”), Evolved UniversalTerrestrial Radio Access Network (“E-UTRAN”), Long-Term Evolution (LTE),and the like.

Transmit front-end 201 is adapted to process and generate transmitsignal A. Signal splitter 210 splits the transmit signal and delivers aportion (sample) of this signal, i.e., signal B, to self-interferencecancellation circuit 390. The remaining portion of the transmit signal,which is relatively large (e.g., 85% of the transmit signal) isdelivered to transmit antenna A11. Because the transmit and receiveantenna A11 and A12 operate in substantially the same frequency band,signal IN received by receive antenna A12 includes the desired signal aswell as a portion of the transmitted signal OUT. The transmitted signalcomponent that is received by antenna A12 is an undesirable signal andis referred to hereinafter as the self-interference signal.Self-interference cancellation circuit 390 operates to reconstruct theself-interference signal, which is subsequently subtracted from thereceived signal IN. To achieve this, self-interference cancellationcircuit 390 generates a multitude of weighted and delayed samples of thetransmit signal, and combine these signals to generate signal C that isrepresentative of the self-interference signal. Signal combiner 310 isadapted to subtract the signal it receives from self-interferencecancellation circuit 390 from the signal it receives from antenna A12,thereby to deliver the resulting signal D to receive front-end 300.Accordingly, the self-interference component of the signal received byreceive front-end 300 is substantially degraded. In one embodiment,self-cancellation circuit 390 may cancel, e.g., 20-25 dB of theself-interference signal.

FIG. 2 is a simplified block diagram of a full-duplex wirelesscommunication device (hereinafter alternatively referred to as device)200, in accordance with another embodiment of the present invention.Device 200 is similar to device 100 except that device 200 has a singleantenna A21 used for both transmission and reception of signals. Device200 also includes a circulator 350 that provides isolation between itsports. Circulator 350 is adapted to concurrently deliver the transmitsignal and the receive signal to and from antenna A21. In one exemplaryembodiment, circulator 350 provides approximately 15 dB of isolationbetween the transmit and receive paths, thereby reducing theself-interference on the receive port by approximately 15 dB.

FIG. 3 is a simplified block diagram of a full-duplex wirelesscommunication device (hereinafter alternatively referred to as device)300, in accordance with one exemplary embodiment of the presentinvention. Device 300 is shown as including a transmitter front end 201,a receiver front end 301, a transmit/receive antenna A31, a circulator350, and a self-interference cancellation circuit 390 as is alsodisposed in devices 100 and 200 shown in FIGS. 1 and 2, respectively.Transmitter front end 201 receives a complex digital signal I(n)+jQ(n),converts the complex digital signal into an analog signal I(f)+jQ(f),and frequency translates the analog signal to a RF transmit signal 205.Coupler 210 receives a sample of transmit signal 205 and in responsedelivers a through signal 212 to circulator 350, and a coupled signal214 to splitter 215. Self-interference signal cancellation circuit 390is adapted to reconstruct the self-interference signal 314 from thesample of the transmit signal 214. The reconstructed self-interferencesignal 314 is subtracted from received signal 218 by coupler 310 therebyto recover the signal of interest 305, also referred to as the desiredsignal. The desired signal 305 is delivered to receiver front end 301for further processing.

Although FIG. 3 is shown as having 8 taps 5 ₁, 5 ₂, 5 ₃, 5 ₄, 5 ₅, 5 ₆,5 ₇, 5 ₈, it is understood that a signal cancellation circuit inaccordance with embodiments of the present invention may have any numberof taps. Signal cancellation circuit 390 is adapted to enable fullduplex wireless communication by canceling or minimizing theself-interference signal received by receiver 301 and caused by signaltransmission at transmitter 201.

Signal cancellation circuit 390 is adapted to receive a sample of thetransmitted signal 205 (self-interference signal) transmitted bytransmitter 201, reconstruct the self-interference signal from thereceived samples, and cancel the reconstructed signal from receivedsignal 305 thereby to recover the signal of interest, also referred toas the desired signal. Signal cancellation circuit 390 is described indetail in US Application No. 61/736,726, the content of which isincorporated herein by reference in its entirety.

In the following, for simplicity, the same reference number may be usedto identify both the path through which a signal travels, as well as tothe signal which travels through that path. For example, referencenumeral 25 may be used to refer to the signal so identified in FIG. 3,or alternatively to the path through which this signal travels.Furthermore, in the following, the terms splitter, coupler, or combinerare alternatively used to refer to an element adapted to split/divide asignal to generate more signals and/or couple/combine a multitude ofsignals to generate one or more signals. Such a component is alsoalternatively referred to herein as splitter/coupler. Furthermore, usingcouplers for signal distribution and combination is only one example. Itis understood that other isolating components or a pair of antennas maybe used instead of a circulator.

Signal cancellation circuit 390 is shown as including, eight variableattenuators 4 ₁, 4 ₂, 4 ₃, 4 ₄, 4 ₅, 4 ₆, 4 ₇, 4 ₈ each adapted toattenuate the signal it receives from its associated delay element inaccordance with a different attenuation signal ATT₁-ATT₈. Accordingly,signals 130, 125, 115, 105, 135, 145, 155, 160 supplied respectively byvariable attenuators 4 ₁, 4 ₂, 4 ₃, 4 ₄, 4 ₅, 4 ₆, 4 ₇, 4 ₈(alternatively and collectively referred to herein using referencenumber 4) are time-delayed, weighted signal samples that are used toreconstruct the self-interference component of the transmitted signal.

In accordance with the present invention, the attenuation signalsATT₁-ATT₈ applied to variable attenuators 4 ₁, 4 ₂, 4 ₃, 4 ₄, 4 ₅, 4 ₆,4 ₇, 4 ₈ are selected so as to maximize the matching between signals 314and 218—respectively supplied by coupler 315 and circulator 350—in orderto achieve maximum cancellation. To achieve this, the cancellation paths(8 in the exemplary embodiment shown in FIG. 3) are first characterizedfor a number of different frequencies. One such characterizationincludes generating a complex response plot (also known as polar plot)of the cancellation circuit after disconnecting the self-interferencepath, i.e., disconnecting path 218 from coupler 310. As is well known, acomplex plot shows the real and imaginary parts of a signal on x-axisand y-axis respectively. One may obtain the complex response plot bydividing the frequency domain representation of the received signal bythe frequency domain representation of the transmitted signal, commonlyknown as the channel response. Alternatively, the received signal may beused without such division if the same transmit signal is used acrossall measurements.

In one embodiment, the attenuation signals applied to the attenuatorsare initially set to a maximum value. Next, signal 314 generated by thecancellation circuit at the output of coupler 315 is measured at a firstfrequency f₁, thus defining a first starting point, as described furtherbelow. Such a measurement is repeated N times at N different frequenciesto generate N starting points each associated with a different one ofthe N frequencies. FIG. 4 shows a plot 400 of the N responses. Exemplarypoints s₁, s₂, and s₃ respectively show the response of the cancellationcircuit at the output of coupler 315 at frequencies f₁, f₂ and f₃. Theresponse at N frequency points can also be measured by transmitting asingle wideband signal and measuring the N-point frequency domainrepresentation of the response.

If the level of attenuation applied to any of the attenuators is variedfrom a maximum value to a minimum value at any given frequency, theresponse of the cancellation circuit is observed as following asubstantially linear path on the complex plane. For example, assume thatat frequency fond with the maximum amount of attenuation applied toattenuator 4 ₁, the starting point (response of the cancellation circuitwith the circulator path disconnected) is represented by s₁. If theattenuation level ATT₁ is subsequently changed from the maximumattention level to a lower attenuation level, the response of thecancellation circuit at frequency f₁ is observed as traversing a linearpath from point s₁ to point p₁₁ defined by:imag{p ₁₁}=imag{s ₁ }+m ₁₁*(real{p ₁₁}−real{s ₁})  (1)Where p₁₁ represents the cancellation circuit's response at frequencyf₁, and m₁₁ represents the slope of the line connecting points s₁ andp₁₁, as shown in FIG. 4.

It is also determined that the complex output response changes linearlywith

$10^{\frac{{ATT}_{i}}{20}},$where ATT_(i) represents the attenuation level applied to attenuator 4_(i) in dB. For a frequency f_(j), the complex output response may thusbe defined as:

$\begin{matrix}{p_{ij} = {s_{j} + {m_{ij}*10^{\frac{{ATT}_{i}}{20}}}}} & (2)\end{matrix}$

Assume that the attenuation levels ATT₁ and ATT₂ are applied toattenuators 4 ₁ and 4 ₂ while the frequency is maintained at f₁. Thesuperposition principle is then used to determine the system's response,as shown below:

$\begin{matrix}{p_{1} = {s_{1} + {m_{11}*10^{\frac{{ATT}_{1}}{20}}} + {m_{21}*10^{\frac{{ATT}_{2}}{20}}}}} & (3)\end{matrix}$

Likewise, assume that the attenuation levels ATT₁ and ATT₂ are appliedto attenuators 4 ₁ and 4 ₂ while the frequency is maintained at f₂. Thesuperposition principle is then used to determine the system's response,as shown below:

$\begin{matrix}{p_{2} = {s_{2} + {m_{12}*10^{\frac{{ATT}_{1}}{20}}} + {m_{22}*10^{\frac{{ATT}_{2}}{20}}}}} & (4)\end{matrix}$

Assuming that the cancellation circuit responses are measured at threedifferent frequencies, namely f₁, f₂, and f₃, and the attenuationlevels, ATT₁-ATT₈ are varied concurrently during each sub-frequency, theresponse of the cancellation circuit may be represented by:P _(f) =M*ATT  (5)

In expression (5), P_(f) is a complex column vector defined by:

$\quad\begin{bmatrix}P_{f\; 1} \\P_{f\; 2} \\P_{f\; 3}\end{bmatrix}$S is a complex column vector defined by:

$\quad\begin{bmatrix}s_{1} \\s_{2} \\s_{3}\end{bmatrix}$M is a complex matrix of size 3×8 defined by:

$\quad\begin{bmatrix}{m_{11}m_{21}\mspace{14mu}\ldots\mspace{14mu} m_{71}m_{81}} \\{m_{12}m_{22}\mspace{14mu}\ldots\mspace{14mu} m_{72}m_{82}} \\{m_{13}m_{23}\mspace{14mu}\ldots\mspace{14mu} m_{73}m_{83}}\end{bmatrix}$And ATT is a real column vector defined by:

$\begin{bmatrix}10^{\frac{{ATT}_{1}}{20}} \\10^{\frac{{ATT}_{2}}{20}} \\\vdots \\10^{\frac{{ATT}_{8}}{20}}\end{bmatrix}.$

Next, the complex response of the self-interference path at these threefrequencies f₁, f₂, and f₃ is measured, i.e., after connecting thecirculator back to coupler 310. Assume the complex interferencemeasurement is defined by:

$I = \begin{bmatrix}I_{f\; 1} \\I_{f\; 2} \\I_{f\; 3}\end{bmatrix}$

This self-interference may be measured, for example, after applyingmaximum attenuation levels to the attenuators. Variable I thusrepresents the superposition of the actual self-interference with theattenuators receiving the attenuation levels leading to cancellationresponse at point S, as described above. The actual self-interference isthus defined by (I−S).

Therefore, the solution to the equation:(I−S)+(S+M*ATT)=0which is the same asI+M*ATT=0  (6)provides the optimum attenuation levels.

The attenuation values obtained from equation (6) are on the continuousreal line and thus are rounded to the nearest attenuation values if stepattenuators are used. This quantization step may lead to aless-than-optimum self-interference signal cancellation and a residualsignal. In another embodiment, discrete algebra may be used to accountfor quantization of the attenuation levels to achieve optimal solutionfor the attenuation levels.

The above description applies to tuning the cancellation circuitinitially. The same principle may also apply to adapting thecancellation circuit to channel changes. Any channel change may lead tonon-ideal conditions leading to a small residual signal.

Assume the residual interference signal is represented by I_(residual).Residual attenuation levels ATT_(residual) may thus be obtained bysolving the following equation:I _(residual) +M*ATT_(residual)=0

The residual attenuation values ATT_(residual) may be subsequently usedto compute new attenuation levels iteratively. Accordingly, this processis iteratively carried out until a predefined condition is satisfied.

The above embodiments of the present invention are illustrative and notlimitative. For example, while the above description is applicable to amethod of tuning a cancellation circuit, it is understood that in otherembodiments, it is possible to measure P_(f) and S without disconnectingthe self-interference path, in which case both P_(f) and S are offset bythe self-interference path, thus resulting in achieving the same matrixM. Embodiments of the present invention are not limited by the number oftaps used in the signal cancellation circuit. Nor is the inventionlimited by the number of frequencies used to measure responses tooptimize the attenuation values. Embodiments of the present inventionare not limited by the type of delay element, attenuator, passivecoupler, splitter, combiner, or the like, used in the cancellationcircuit. Although the above description is provided with reference to amulti-tap feed-forward cancellation circuit, it is understood that theabove descriptions of the present invention are equally applicable to amulti-tap feedback cancellation circuit.

FIG. 5 shows a flowchart of a method 500 for canceling or reducing theself-interference signal at a receiver of a communication device, inaccordance with one embodiment of the present invention. To achievethis, a sample of the transmit signal at a first frequency is suppliedto the cancellation circuit at 510. Thereafter, the controller sets afirst attenuation level to one or more variable attenuators at 520. Afirst output signal is obtained at the output of the cancellationcircuit by combining output signals from all the signal paths at 530. Inan embodiment, the first attenuation level may be applied to allattenuators. Next, the controller set a second attenuation level to theone or more cancellation attenuators at 540. In an embodiment, thesecond attenuation level may be applied to all attenuators. In anembodiment, the first attenuation may be the maximum attenuation leveland the second attenuation level may be the minimum attenuation level.Thereafter, a second output signal is obtained at the output of thecancellation circuit by combining output signals from all the signalpaths at 550. Note that transmit signal is a complex signal, thus, theoutput signals are also complex signals. Next, at 560 the communicationdevice generates a matrix based on the first and second output signalsthat are obtained with the corresponding first and second attenuationlevels. For example, the device may compute the matrix by solvingEquation (1) or determine the matrix graphically using the plot of FIG.4. The device may iteratively repeats the above steps 510 to 540 togenerate and expand the matrix by supplying a sample of the transmitsignal at a second frequency, at a third frequency, etc. In anembodiment, the matrix is determined or computed with the receiver beingdisconnected from the antenna or from the circulator. Next, a signal isreceived at the receiver at 570. The controller concurrently varies theattenuation level of each signal path so that the product of the matrixand the attenuation levels of the signal paths is substantially equal tothe received signal at 580. In an embodiment, these thus obtainedattenuation levels are applied to the signal paths, whose outputs arecombined by combiner 315 to generate a signal representative of theself-interference signal. The reconstructed signal is subsequentlysubtracted from the received signal to cancel or reduce theself-interference signal at the receiver.

The above embodiments of the present invention are illustrative and notlimitative. Embodiments of the present invention are not limited by thenumber of taps used in the signal cancellation circuit. Embodiments ofthe present invention are not limited by the type of delay element,attenuator, passive coupler, splitter, combiner, amplifier, or the like,used in the cancellation circuit. Embodiments of the present inventionare not limited by the number of antennas used in a full-duplex wirelesscommunication device. Embodiments of the present invention are notlimited by the frequency of transmission or reception of the signal.Embodiment of the present invention are not limited by the type ornumber of substrates, semiconductor or otherwise, used to from afull-duplex wireless communication device. Other additions,subtractions, or modifications are obvious in view of the presentdisclosure and are intended to fall within the scope of the appendedclaims.

What is claimed is:
 1. A circuit comprising: a transmitter thattransmits a transmit signal; a plurality of signal paths, each signalpath including a delay element and a variable attenuator; wherein eachsignal path receives a portion of the transmit signal; a control unitthat sets one or more signal paths to a first attenuation level and asecond attenuation level; a combiner that generates an output signal bycombining outputs of the plurality of signal paths; and a receiver thatreceives a signal, wherein the circuit computes a matrix based on firstand second output signals associated with the respective first andsecond attenuation levels, and wherein the control unit concurrentlyvaries an attenuation level of each signal path so that a product of thematrix and the attenuation level of each signal path is substantiallyequal to the received signal; wherein the matrix is determined byapplying sequentially a number of transmit signals having differentfrequencies to the signal paths; wherein the matrix is a complex M×Nmatrix, M and N being positive integers, where M is the number of thedifferent frequencies, and N is the number of the signal paths; whereinthe received signal is a complex M×1 column vector and the attenuationlevel of each signal path is an element of a real N×1 column vector. 2.The circuit of claim 1, further comprising: at least one antenna forreceiving the signal.
 3. The circuit of claim 2, wherein each of thesignal paths receives a sample of the transmit signal and generates adelayed and weighted sample of the transmit signal.
 4. The circuit ofclaim 3, further comprising: a plurality of passive couplers that changea phase of one or more delayed and weighted signal paths in relation tothe transmit signal; and a coupler that subtracts the product from areceived signal.
 5. The circuit of claim 1, wherein the matrix isdetermined with a fast Fourier transform (FFT) of a single widebandtransmit signal.
 6. The circuit of claim 1, further comprising acirculator having a first port coupled to an antenna, a second portcoupled to a transmit line of the circuit, and a third port coupled to areceive line of the circuit.
 7. The circuit of claim 6, wherein thethird port of the circulator is disconnected from the receiver while thecircuit is computing the matrix.
 8. The circuit of claim 7, wherein thethird port of the circulator is connected to the receiver while thecontrol unit concurrently varies the attenuation level of each signalpath to minimize the sum of the product and the received signal.
 9. Thecircuit of claim 1, wherein the first attenuation level is a maximumattenuation level and the second attenuation level is a minimumattenuation level.
 10. The circuit of claim 1, wherein the control unititeratively varies the attenuation level of each variable attenuatoruntil a sum of the product and the received signal satisfies apredefined condition.
 11. A method for performing a self-interferencesignal cancellation in a full-duplex wireless communication system witha cancellation circuit having a plurality of parallel signal paths, eachof the signal paths including a delay element and a variable attenuator,the method comprising: supplying a first sample of a transmit signal ata first frequency by a transmitter to the cancellation circuit; settinga first attenuation level to one or more of the variable attenuators;obtaining a first output signal of the cancellation circuit; setting asecond attenuation level to the one or more of the variable attenuators;obtaining a second output signal of the cancellation circuit; generatinga matrix based on the first and second output signals; receiving asignal at a receiver; and varying an attenuation level of each of thevariable attenuators so that a product of the matrix and the attenuationlevels of the variable attenuators is substantially equal to thereceived signal; wherein generating a matrix comprises iterativelyperforming the following steps: supplying a second sample of thetransmit signal at a second frequency by the transmitter to thecancellation circuit; setting the first attenuation level to the one ormore of the variable attenuators; obtaining a third output signal of thecancellation circuit; setting the second attenuation level to the one ormore of the variable attenuators; obtaining a fourth output signal ofthe cancellation circuit; and expanding the matrix based on the thirdand fourth output signals.
 12. The method of claim 11, wherein varyingan attenuation level of each of the variable attenuators is performedconcurrently.
 13. The method of claim 11, wherein the matrix is acomplex M×N matrix, M and N being positive integers, where M is a numberof different frequencies of the transmit signal and N is a number of theplurality of signal paths.
 14. The method of claim 11, whereingenerating a matrix is without connecting the transmitter to an antenna.15. The method of claim 11, wherein the received signal at the receiveris through an antenna while a maximum attenuation level is applied tothe attenuators.
 16. The method of claim 11, wherein the first outputsignal or the second output signal is obtained by combining outputs fromall of the plurality of signal paths.
 17. The method of claim 11,wherein the first attenuation level is a maximum attenuation.
 18. Themethod of claim 11, wherein the second attenuation level is a minimumattenuation.
 19. The method of claim 11, wherein the first and secondattenuation levels are applied to all variable attenuators of theplurality of parallel signal paths.
 20. The method of claim 11, whereinthe transmit signal is a complex signal.
 21. The method of claim 11,wherein varying an attenuation level of each of the variable attenuatorscomprises iteratively carrying out a tuning of the attenuation of eachof the variable attenuators until a sum of the product and the receivedsignal satisfies a predefined condition.
 22. A circuit comprising: atransmitter that transmits a transmit signal; a plurality of signalpaths, each signal path including a delay element and a variableattenuator; wherein each signal path receives a portion of the transmitsignal; a control unit that sets one or more signal paths to a firstattenuation level and a second attenuation level; a combiner thatgenerates an output signal by combining outputs of the plurality ofsignal paths; a circulator having a first port coupled to an antenna, asecond port coupled to a transmit line of the circuit, and a third portcoupled to a receive line of the circuit; and a receiver that receives asignal, wherein the circuit computes a matrix based on first and secondoutput signals associated with the respective first and secondattenuation levels, and wherein the control unit concurrently varies anattenuation level of each signal path so that a product of the matrixand the attenuation level of each signal path is substantially equal tothe received signal; wherein the third port of the circulator isdisconnected from the receiver while the circuit is computing thematrix; wherein the third port of the circulator is connected to thereceiver while the control unit concurrently varies the attenuationlevel of each signal path to minimize the sum of the product and thereceived signal.
 23. The circuit of claim 22, further comprising: atleast one antenna for receiving the signal.
 24. The circuit of claim 23,wherein each of the signal paths receives a sample of the transmitsignal and generates a delayed and weighted sample of the transmitsignal.
 25. The circuit of claim 24, further comprising: a plurality ofpassive couplers that change a phase of one or more delayed and weightedsignal paths in relation to the transmit signal; and a coupler thatsubtracts the product from a received signal.
 26. The circuit of claim22, wherein the matrix is determined with a fast Fourier transform (FFT)of a single wideband transmit signal.
 27. The circuit of claim 22,wherein the first attenuation level is a maximum attenuation level andthe second attenuation level is a minimum attenuation level.
 28. Thecircuit of claim 22, wherein the control unit iteratively varies theattenuation level of each variable attenuator until a sum of the productand the received signal satisfies a predefined condition.